This invention relates to rotary artificial hearts which utilize centrifugal-flow, mixed-flow, or axial-flow pumps, principally driven by electric motors. Many such pumps are either in use for temporary blood pumping, such as with the heart lung machine, or under investigation for long-term use, such as those disclosed in prior U.S. Pat. Nos. 4,994,078 and 5,092,879 by the present inventor. Intraventricular axial-flow-pump, left-heart assist devices as disclosed in these patents have been successfully implanted in experimental animals for months with negligible blood damage, and have functioned in bench durability tests for years. The ultimate goal is to support human patients literally for decades, and although the use of properly designed mechanical radial bearings may accomplish this, magnetic suspension has the advantage of reduced wear with the greatest long-term durability potential.
Previous attempts to accomplish magnetic suspension in artificial hearts have required position sensors and electromagnets to stabilize at least one degree of freedom of the rotor. U.S. Pat. Nos. 4,688,998 by Olsen et al., 4,763,032 by Bramm et al., and 4,779,614 by Moise, each provide full, three-dimensional magnetic suspension of the rotor with electromagnetically-actuated rotor position adjustment with feedback control based on sensing motion of the rotor, either out of radial center or out of optimal axial position. The disadvantages of electromagnetically-actuated feedback control include the need for electromagnetic actuators, sensors and control circuitry, all of which add volume, weight, and complexity to the device and consume electric power. The present invention retains the rotor in optimal radial and axial position at all times. No sensors are used to measure rotor malposition because malposition never occurs. The rotor is suspended radially, and magnetically biased in one axial direction. Axial position is held by a single-point contact thrust bearing, and all thrust forces on the rotor, including those resulting from the hydrodynamic interaction of the impeller with the blood, are maintained below the force required to displace the thrust bearing from contact. Furthermore, when thrust balancing of the pump impellers is used, force on the thrust bearing is minimized. Since the thrust-bearing contact point is at the center of rotation, surface friction, wear, and heat generation are also minimized. Also, the thrust bearing point is located in a high-flow position for sufficient washing to prevent thrombus accumulation.
The magnetic bearing of the present invention may be used with centrifugal pumps and may also incorporate mechanical radial position limiters to maintain alignment of the rotor in nearly centered radial position if transient forces momentarily overcome the magnetic radial bearing capacity. Such mechanical limiters may take the form of mechanical radial bearings having a large enough radial clearance between the stationary and rotating members so that in usual operation they do not support the radial load and do not wear. In axial pump configurations, the impeller blades may be configured such that the rotor is mechanically supported by contact between the impeller tips and the housing within which the impeller rotates during transient conditions where radial load exceeds the magnetic bearing capacity. The blades may be composed of wear-resistant materials, such as ceramic, or may utilize wear-resistant inserts, and the housing bore in proximity to the blades may also be fabricated of wear-resistant materials. Thus, even with occasional mechanical contact, no galling of the surfaces or other damage to the pump will occur.